Spindle motor

ABSTRACT

A spindle motor comprising a bracket, a shaft member provided on the bracket, a rotor rotatably mounted to the shaft member through a pair of bearings, a rotor magnet mounted to the rotors, a stator disposed in opposite relation to the rotor magnet, and magnetic fluid sealing means respectively disposed outwardly of a pair of the bearings. The shaft member is provided with an air passage extending in the axial direction, and a communication passage for communicating the air passage with a space between a pair of the bearings. A cap member which is improved in its structure is disposed outwardly of one of the paired bearings. An annular projection for positioning the rotor magnet concentrically with respect to the rotor is provided on a magnet mount portion of the rotor.

This is a divisional of application Ser. No. 08/004,098 filed on Jan.15, 1993, now U.S. Pat. No. 5,396,134.

FIELD OF THE INVENTION

The present invention relates to a spindle motor for driving a recordingdisk such as a magnetic disk to rotate.

PRIOR ART OF THE INVENTION

As disclosed in U.S. Pat. No. 5,047,677, for example, a spindle motorgenerally comprises a bracket mounted to a disk drive, a shaft :memberprovided on the bracket, a rotor rotatably mounted to the shaft memberthrough a pair of bearings, a rotor magnet mounted to the rotor, and astator disposed in opposite relation to the rotor magnet. A magneticfluid sealing means is disposed outwardly of a pair of the bearings (onthe same side as a free end of the shaft member) and serves to preventgrease used in the bearings from scattering to the outside. Also, alabyrinth seal means is disposed between the bracket and the rotor andserves to prevent the grease in the bearings from scattering to theoutside through therebetween.

However, such a spindle motor has the drawback that because thelabyrinth seal means cannot positively seal between the inside and theoutside of the motor, some amount of grease inevitably scatters to theoutside of the motor through the labyrinth seal means.

In order to eliminate the above-mentioned drawback, it could be thoughtto arrange another magnetic fluid sealing means on the opposite side ofa pair of the bearings (i.e., on the side nearer to a base end of theshaft member) as well. With this arrangement, however, the gap betweenthe shaft member and the rotor is enclosed by a magnetic fluid, hencethe space between both the sealing means is completely sealed.Accordingly, upon a change in temperature, i.e., upon being subjected toa high temperature, there gives rise to a problem that air in the spacebetween both the sealing means is expanded and the magnetic fluidscatters outward from both the sealing means. This phenomenon alsooccurs when the pressure within the disk drive is lowered due to achange in atmospheric pressure.

Further, in the case of using the magnetic fluid sealing means as a sealmeans like the motor disclosed in the above-cited U.S. Pat. No.5,047,677, a cap member for preventing scatter of time magnetic fluid ispreferably disposed outwardly of the magnetic fluid sealing means asdisclosed in U.S. Pat. No. 5,009,436, for example.

The provision of such a cap member, however, gives rise to the followingproblem. More specifically, in the case where the cap member is disposedto directly pile over a magnetic fluid holding means of the magneticfluid sealing means as illustrated in FIGS. 8, 9 and 11 of theabove-cited U.S. Pat. No. 5,009,436, for example, an adhesive for fixingthe magnetic fluid holding means and the cap member is not sufficientlyapplied to the magnetic fluid holding means on the lower side. As aresult, the magnetic fluid holding means cannot be fixed in a sufficientand positive manner. On the other hand, in the case where the cap memberis disposed to be spaced from the magnetic fluid holding means asillustrated in FIGS. 10 and 12 of the above-cited U.S. Pat. No.5,009,436, for example, an adhesive must be separately applied to themagnetic fluid holding means and the cap member and, as a result, anoperation of applying the adhesive becomes troublesome.

Moreover, the rotor magnet is fixed directly or via a yoke to the rotorby using an adhesive as disclosed in the above-cited U.S. Pat. No.5,047,677 or U.S. Pat. No. 5,045,738, for example.

But it is so difficult to apply the adhesive over the innercircumferential surface of the rotor or the yoke with an essentiallyuniform thickness that the rotor magnet may be eccentrically mounted tothe rotor or the yoke. As a result, the gap between the stator and therotor magnet may not become uniform, thus causing unevenness in rotationof the rotor.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a spindle motor inwhich a magnetic fluid can be positively prevented from scattering evenwhen magnetic fluid sealing means are provided on both sides of a pairof bearings.

A second object of the present invention is to provide a spindle motorin which the magnetic fluid sealing means and a cap member can be easilyand positively fixed by using an adhesive.

A third object of the present invention is to provide a spindle motor inwhich a rotor magnet can be concentrically mounted to a magnet mountportion of a rotor.

Other objects and features of the present invention will be easilyunderstood from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing one embodiment of a spindle motoraccording to the present invention.

FIG. 2 is a sectional view taken along line II--II in FIG. 1.

FIG. 3 is a partial enlarged sectional view showing a pair of bearingsand thereabout in the spindle motor of FIG. 1.

FIG. 4 is a top plan view showing a cap member in the spindle motor ofFIG. 1.

FIG. 5 is a fragmentary perspective view, partially sectioned, of thecap member in FIG. 4.

FIG. 6 is a sectional view showing a part of a rotor in the spindlemotor of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, one preferred embodiment of a spindle motor according tothe present invention will be described with reference to the attacheddrawings.

An illustrated spindle motor 1 includes a substantially circular bracket2 mounted to a disk drive, and a rotor 3 rotatable relative to thebracket 2. The bracket 2 comprises a cylindrical bracket body 4 with thebottom, a boss portion 5 positioned at the center of the bracket body 4,and a flange 6 extending outwardly from the outer circumference of thebracket body 4 at its upper end.

A shaft member 7 made of magnetic material is fixedly press-fitted intothe boss portion 5, and a penetration hole 8 is formed to axially extendthrough the center of the shaft member 8 over its entire length. Thepenetration hole 8 has one end which is open at the under surface of thebracket 2, and the other end provided with a female-threaded fixtureportion 9 through which the free end of the shaft member is fixed to acover of the disk drive by screw fitting.

A pair of ball bearings 10, 11 are secured to the outer circumference ofthe shaft member 7 in its upper portion so that the rotor 3 is rotatablysupported through a pair of the bail bearings 10, 11. At a position ofthe shaft member 7 facing a space 12 between both the ball bearings, asshown in FIG. 2, there are formed four radial communication holes 13with equal angular intervals for communicating the space 12 between boththe ball bearings with the penetration hole 8. Also, an elastic ball 14is inserted in the penetration hole 8 at a position intermediate thefemale-threaded fixture portion 9 and the communication holes 13. Theelastic ball 14 which constitutes a seal means can be formed ofsynthetic rubber material, for example. By so providing the elastic ball14, the communication of the space 12 between a pair of the ballbearings 10, 11 with a disk chamber (which accommodates the rotor 3 andthe bracket 2 at its upper surface) is cut off, while the space 12 isheld in communication with the outside of the motor (i.e., the outsideof the disk chamber) via the communication holes 13 and the penetrationhole 8 (these holes constituting an air passage).

Outwardly of both the ball bearings 10, 11 between the shaft member 7and the rotor 3, there are mounted magnetic fluid sealing means 15, 16via bushings 17, 18, respectively.

The sealing means 15, 16 respectively include magnetic fluid holdingmeans 15e, 15e which comprise, as shown in FIG. 3, ring-shaped permanentmagnets 15a, 16a magnetized in the axial direction and pairs of polepieces 15b, 15c ; 16b, 16c attached to the upper and lower surfaces ofthe permanent magnets. Magnetic fluids 15d, 16d are injected torespective gaps between the pole pieces 15b, 15c, 16b, 16c and the shaftmember 7, whereby a space between both the magnetic fluid sealing means15 and 16 is sealed. A cap member 19 is attached to the upper surface ofthe magnetic fluid sealing means 15 on the upper side. The cap member 19will be described later.

The rotor 3 comprises a cylindrical hub portion 20, a flange portion 21provided at a lower end of the hub portion 20, and a rotor portion 22extending downwardly from the outer circumference of the flange portion21. The hub portion 20 is supported to the shaft member through both theball bearings 10, 11 and a plurality of recording disks (not shown) arefitted over the outer circumference of the hub portion 20. The recordingdisks are fixedly held in sandwich relation between the flange portion21 and a clamp member (not shown) fitted from above. The clamp member issecured in place by screws respectively screwed into partly threadedholes 23 of the hub portion 20.

An annular rotor magnet 24 is fixed to the inner circumferential surfaceof the rotor portion 22. A stator 25 is disposed in opposite relation tothe rotor magnet 24 and fixed to the outer circumference of the bossportion 5 of the bracket 2. A manner of mounting the rotor magnet 24will be described later.

With the spindle motor thus constructed, since the space 12 between boththe ball bearings 10 and 11 is held open to the underside of the bracket2, i.e., the outside of the disk chamber, via the communication holes 13and the penetration hole 8, the space between the magnetic fluid sealingmeans 15 and 16 is not isolated from the outside so that, when an airpressure in the space 12 rises (or lowers), air is discharged to theoutside (or introduced from the outside) via the communication holes 13and the penetration hole 8. Accordingly, upon a rise in temperature or achange in pressure, the air pressure between both the magnetic fluidsealing means 15 and 16 is essentially subjected to no variations, hencethe magnetic fluid is positively prevented from scattering.

Furthermore, neither special structure nor difficult machining arerequired to communicate the space between both the magnetic fluidsealing means 15 and 16 with the outside. Thus, it is only needed tomake holes in the shaft member 7 and insert the elastic ball 14, meaningthat manufacture of the spindle motor is very simple.

Additionally, since the female-threaded fixture portion 9 can be formedby utilizing the other end portion of the penetration hole 8, it ispossible to easily provide the female-threaded :fixture portion 9 forfixing the shaft member 7.

The cap member 19 and a manner of mounting the same will be explainedbelow by referring to FIGS. 4 and 5 along with FIG. 2.

The illustrated cap member 19 comprises an annular support portion 19ahaving a substantially U-shaped cross-section, a scatter preventingportion 19b provided at the inner circumference of the annular supportportion 19a and projecting inwardly therefrom in the radial direction,and an attachment portion 19c provided at the outer circumference of theannular support portion 19a and extending outwardly therefrom in theradial direction. In the illustrated embodiments, there are formed aplurality of (for example, twelve) cut-outs 19d of relatively large sizewith equal intervals in the circumferential direction, each cutout 19dbeing extended from an outer wall of the support portion 19a to theattachment portion 19c of the cap member 19 while leaving rectangularapertures therein. Although the cutouts 19d are each shown as extendingfrom the support portion 19a to the attachment portion 19c for easierapplication of an adhesive 30 described later, a desired effect can alsobe achieved by forming the cut-outs in either the support portion 19a orthe attachment portion 19c. However, if the cut-outs 19d are relativelysmall in size, it would be difficult to apply the adhesive 30.

The bushing 17 has a substantially L-shaped cross-section and includesan annular stepped portion 17a on the lower side and an annular shoulderportion 17b on the upper side. The outer circumferential edge of themagnetic fluid holding means 15e is positioned over the relatively largestepped portion 17a, and the attachment portion 19c of the cap member 19is positioned over the relatively small shoulder portion 17b.

The manner of mounting the cap member 19 will be explained below.

First, the magnetic fluid holding means 15e is inserted from above suchthat its outer circumferential edge is positioned over the steppedportion 17a of the bushing 17. By so inserting, the outercircumferential surface of the magnetic fluid holding means 15e isbrought into contact with the circumferential surface which defines thestepped portion 17a of the bushing 17, and the magnetic fluid holdingmeans 15e is concentrically positioned with respect to the bushing 17.

Then, the cap member 19 is inserted from above. Upon this insertion, thelower surface of the support portion 19a of the cap member 19 is broughtinto contact with the upper surface of the magnetic fluid holding means15e (more exactly, of the pole pieces 15b on the upper side), and theattachment portion 19c of the cap member 19 is positioned over theshoulder portion 17b of the bushing 17 such that the outercircumferential surface of the attachment portion 19c is brought intocontact with the circumferential surface which defines the shoulderportion 17b of the bushing 17. As a result, the cap member 19 isconcentrically positioned with respect to the bushing 17.

After that, the adhesive 30 of ultraviolet setting type is applied fromabove to the substantially U-shaped support portion 19a, the attachmentportion 19c and the cut-outs 19d of the cap member 19. By so applying,the adhesive 30 is allowed to spread over the upper side of thesubstantially U-shaped support portion 19a and a base region of theattachment portion 19c, as well as over the upper surface of an outercircumferential portion of the upper pole piece 15b in the magneticfluid holding means 15e and throughout the contiguous innercircumferential surface of the bushing 17 through the cut-outs 19d.Then, by irradiating a ultraviolet ray to the adhesive 30 from above thecap member 19, the ultraviolet ray is allowed to reach not only theadhesive 30 over the substantially U-shaped support portion 19a and theattachment portion 19c, not only the adhesive 30 below the substantiallyU-shaped support portion 19a and the attachment portion 19c through thecut-outs 19d, thereby setting or hardening the adhesive 30. As a result,the magnetic fluid holding means 15e, the cap member 19 and the innercircumferential surface of the bushing 17 are fixedly bonded together bythe adhesive 30.

Thus, the magnetic fluid holding means 15e and the cap member 19 can besecured in place by a single operation of applying the adhesive, makingit possible to provide higher manufacture efficiency than in the priorart.

It should be noted that although in the illustrated embodiment themagnetic fluid holding means 15e and the cap member 19 are fitted to thebushing 17 which is in turn mounted to the rotor 3, the cap member 19may be attached directly to the rotor 3 while mounting the magneticfluid holding means 15e to the bushing 17. As an alternative, thebushing 17 may be omitted to attach both the magnetic fluid holdingmeans 15e and the cap member 19 directly to the rotor 3.

The manner of mounting the rotor magnet 24 will be described below byreferring to FIG. 6 along with FIG. 1. The inner circumferential surfaceof the rotor portion 22 serves as a magnet mount portion. At the innerbottom of the rotor portion 22, there is defined a abutting bottomsurface 32 which comes into abutment with the rotor magnet 24 to beinserted. Also, an annular projection 34 is provided on the innercircumferential surface of the rotor portion 22 near the inner bottom.Although the projection 34 is provided one in the illustratedembodiment, it may be provided two or more. The projection 34 serves tomake smaller a contact area between the rotor portion 22 and the rotormagnet 24 for preventing damage of the rotor magnet 24, as well as toposition the rotor magnet 24 concentrically with respect to the rotorportion 22 when the rotor magnet is fitted. In addition, an adhesive pit36 is formed between the abutting bottom surface 32 and the projection34.

The rotor magnet 24 is inserted to the rotor portion 22 with an adhesiveapplied beforehand over the inner circumferential surface of the rotorportion, and fixed to the rotor 31 by being press-fitted with theprojection 34. Since a gap 38 is formed between the rotor magnet 24 andthe rotor portion 22 due to the presence of the projection 34, theadhesive is allowed to stay and set in the gap 38 and the adhesive pit36 without entering between a distal end of the projection 34 and therotor magnet 24.

Accordingly, the rotor magnet 24 is assembled in concentric relation tothe rotor portion 22 and a thickness of the adhesive is made almostuniform in the gap 38, with the result of that rotation of the rotor 3can be more balanced than in the prior art. Further, because of theadhesive having an almost uniform thickness throughout the innercircumference of the rotor portion 22, the rotor magnet 24 can beexactly and positively mounted in the rotor portion 22.

The annular projection 34 serving to position the rotor magnet 24 isprovided on the rotor portion 22 in the illustrated embodiment, but tothe contrary, it may be provided on the outer circumferential surface ofthe rotor magnet 24. In this case, for the purpose of preventing damageof the rotor magnet 24, the annular projection 34 is preferably providedin a rear end portion of the rotor magnet as viewed in the direction ofits insertion.

Also, although the rotor magnet 24 is directly mounted to the rotorportion 22 in the illustrated embodiment, that arrangement should not beregarded as limiting and the rotor magnet may be mounted to the rotorportion 22 via a yoke. In this case, such a yoke constitutes a part ofthe rotor portion 22.

It should be noted that one embodiment of the spindle motor according tothe present invention has been described above, but the presentinvention is not limited to the illustrated embodiment and variouschanges and modifications may be made without departing from the scopeof the invention as set forth in the following claims.

We claim:
 1. A spindle motor comprising a bracket, a rotor rotatablymounted relative to said bracket, bearing means disposed between saidrotor and said bracket, a rotor magnet mounted to said rotor, a statordisposed in opposite relation to said rotor magnet, a magnetic fluidsealing means disposed outwardly of said bearing means, and a cap memberdisposed further outwardly of said magnetic fluid sealing means,wherein:said cap member comprises an annular support portion having asubstantially U-shaped cross-section, an attachment portion extendingoutwardly from the outer circumferential edge of said support portion inthe radial direction, and a scatter preventing portion extendinginwardly from the inner circumferential edge of said support portion inthe radial direction, said attachment portion being mounted to saidrotor, said support portion being supported to said magnetic fluidsealing means, a cut-out is formed from said support portion to saidattachment portion of said cap member, whereby an adhesive for fixingsaid cap member is applied to spread from the outer side of said supportportion to the inner side of said attachment portion through saidcut-out.
 2. A spindle motor according to claim 1, wherein said cut-outis extended from a part of said support portion to an edge of said capmember.
 3. A spindle motor according to claim 2, wherein said cut-out isformed plural in number at equal intervals in the circumferentialdirection of said cap member.